This is a natural history and a clinical outcomes study in patients who initially present with an ACS. The natural history portion of the study is designed to describe the temporal progression of atherosclerosis in segments of coronary arteries with low ESS and expansive remodeling using intracoronary vascular profiling techniques utilizing intravascular ultrasound (IVUS) and coronary angiography. The clinical outcomes portion of the study is designed to evaluate the efficacy of coronary vascular profiling to predict segments of coronary arteries that will become areas of rapid plaque growth or rupture leading to recurrent major clinical coronary events.